1. Field of the Invention
This invention, in the production of a porous material, preferably a porous material having continuous pores formed in the surface and the interior thereof and consequently containing continued cells (hereinafter occasionally referred to as "open cells") by polymerizing a water in oil type high internal phase emulsion (hereinafter occasionally referred to briefly as "HIPE"), relates to a method for the production of a porous material by the continuous operation from the step of supplying the HIPE through the step of polymerizing it. More particularly, this invention relates to a method for the production of a porous material by the continuous operation from the step of supplying the HIPE through the step of polymerizing it and in which the porous material can be extensively utilized for (1) liquid absorbent materials such as, for example, 1 core materials to be used in disposable diapers for absorbing water, urine, and other excretions and 2 waste oil treating agents and waste solvent treating agents to be used for absorbing oils and organic solvents; (2) energy absorbent materials such as, for example, sound absorbers and heat absorbers in automobiles and buildings for absorbing sound and heat; and (3) chemical impregnating substrates such as, for example, toiletry products impregnated with a flavoring agent, a detergent, a glossing agent, a surface protecting agent, a flame retardant, and the like.
2. Description of the Related Art
The term "HIPE" refers to an emulsion comprising a water phase, i.e. a dispersing phase (internal phase), and an oil phase, i.e. an external phase, and at a W/O ratio of not less than about 3/1. It has been known to produce a porous material from the HIPE (hereinafter occasionally abbreviated simply as the "HIPE method").
The HIPE method is an excellent process for producing a low-density foam of open cells having minute diameters, as compared with the porous material which is produced by using a foaming agent (hereinafter occasionally abbreviated as the "foam") and which is liable to produce a foam of independent cells having comparatively large diameters.
Methods for producing a foam from the HIPE have been disclosed in U.S. Pat. No. 4,522,953, U.S. Pat. No. 4,788,225, U.S. Pat. No. 5,252,619, and U.S. Pat. No. 5,189,070, for example.
A method which comprises forming an HIPE containing a water-soluble and/or oil-soluble initiator and thermally polymerizing the HIPE at 50.degree. C. or 60.degree. C. for 8 hours to 72 hours has been disclosed in U.S. Pat. No. 4,522,953 and U.S. Pat. No. 4,788,225. Further, a method which comprises forming an HIPE, subsequently adding a polymerization initiator thereto, and polymerizing it for four to eight hours has been disclosed in U.S. Pat. No. 5,210,104. Furthermore, a method which comprises forming an HIPE containing an initiator and then polymerizing the HIPE at 100.degree. C. or at a temperature approximating closely thereto thereby decreasing the polymerization time to a level in the range of 3to 5hours has been disclosed in U.S. Pat. No. 5,252,619 and U.S. Pat. No. 5,189,070.
The methods disclosed in U.S. Pat. No. 4,522,953 and U.S. Pat. No. 4,788,225 require an unduly long polymerization time and suffer from deficiency in efficiency of production. Although the methods disclosed in U.S. Pat. No. 5,252,619 and U.S. Pat. No. 5,189,070 enable the polymerization time to be decreased by using the high polymerization temperature, they entail a possibility of requiring the polymerization to continue for several hours, moreover impairing the stability of the HIPE, inducing the liberation of water, and failing to afford a porous material having an expected pore diameter.
According to the method disclosed in U.S. Pat. No. 5,210,104, although the emulsion stability of the HIPE can be improved because the HIPE is formed in the absence of a polymerization initiator and the addition of the polymerization initiator is effected after the formation of the HIPE, the polymerization time is required to last for several hours.
It is an object of this invention to develop a method for the production of a porous material which can complete the polymerization in a briefer time than conventional techniques without impairing the stability of the HIPE.
For the purpose of decreasing the polymerization time, it suffices to increase the decomposition rate of the initiator and consequently heighten the concentration of radicals in the polymerization system. Since an initiator combining an oxidizing agent and a reducing agent (redox initiator) quickly decomposes at a relatively low temperature, the use of the redox initiator for the polymerization of an HIPE can be expected to complete the polymerization in a brief time even at a low temperature sufficient not to impair the stability of the HIPE. An attempt to form an HIPE containing the redox initiator and polymerize this HIPE in such a briefer time than conventional techniques as, for example, within one hour has been ascertained to encounter several problems.
An HIPE can be obtained by stirring and emulsifying an oil phase with a water phase at a temperature in the approximate range of room temperature to 80.degree. C. for a prescribed time. The difference between the temperature of the HIPE thus produced and the polymerization temperature may be preferably as small as possible. To be more specific, an attempt to heighten a low temperature of an HIPE to a higher polymerization temperature possibly elongates the time required for the temperature elevation to such an extent as of lowering the productivity. An attempt to heat the HIPE suddenly in a brief time possibly results in impairing the stability of the HIPE and degrading the quality of the produced porous material. On the other hand, an attempt to emulsify an HIPE at a temperature approximating closely to a polymerization temperature thereof may entail a clear phenomenon of giving rise to a polymerizing reaction of the HIPE and inducing the HIPE to solidify in an emulsifying device during the course of the emulsification, and thus suffering the HIPE to emulsify insufficiently.